Maintaining the structural integrity of certain structures is important in many fields because of safety concerns, downtime, cost, etc. Loss of structural integrity may be caused by material defects, such as cracks, delaminations, disbonds, corrosion, inclusions, voids, etc., that may exist in the structure. For example, it is important in the power generation industry that reliable techniques are available to examine the structural integrity of turbine, generator and associated balance of plant equipment to ensure the components and systems do not suffer failure during operation. A common method for detection of a crack or defect is visual examination by skilled personnel. However, it is known that cracks or defects that may affect the integrity of structural components may not be readily visible without the use of special techniques to aid the examiner. Therefore, various techniques have been developed in the art for the non-invasive and non-destructive analysis of different structural components and materials.
One known technique for the non-invasive and non-destructive analysis of a material for defects comprises thermal imaging where heat is generated in the material and is emitted as radiation in the infrared wavelengths. The location of certain types of defects may be identified as surface temperature variations, where the area of a defect has a temperature differential relative the surrounding area of the material. One implementation of thermal imaging comprises acoustic thermal imaging in which an ultrasonic excitation is used to generate heat in the material. In acoustic thermal imaging, an acoustic thermal effect occurs when sound waves propagate through a solid body or component that contains a crack or other defect causing it to vibrate. Because the faces of the crack ordinarily do not vibrate in unison as the sound waves pass, dissipative phenomena, such as friction between the faces, will convert some of the vibrational energy to heat. By combining this heating effect with infrared imaging, an efficient, non-destructive crack detection system can be realized.
The amount of energy transferred into the material for acoustic thermal imaging may vary, depending on the location and effectiveness of a coupling between an ultrasonic energy and a component being inspected as well as other factors and operating parameters. The amount of energy transferred into a component for acoustic thermal imaging may affect the validity, consistency and/or accuracy of the inspection process for individual components, and in a comparison of plural similar components.